1. Technical Field
The present invention relates to an apparatus for and a method of controlling a gap between calender rolls.
2. Background Art
A calender which rolls a raw material such as resin and rubber between calender rolls should be controlled on a bank formed by the raw material between a roll gap. One of methods for controlling the bank is disclosed in Japanese Patent Application, published Nov. 5, 1977 with Publication No. 52-132072, entitled "Method of Adjusting Amount of Bank in Shaping by Calender". FIG. 3 of the accompanying drawings schematically illustrates the teaching of this disclosure.
As seen from FIG. 3, a bank 4 formed by a raw material 8 fed to a gap 3 between two calender rolls 1 and 2 is irradiated by a light 5 and scanned by a camera 6 to calculate a volume of bank 4. This calculation is used to control the supply of the raw material 8 which is carried by a conveyor 7. It is known that the thickness of products changes if the supply of the raw material changes, and this method maintains the volume of bank 4 at a predetermined value to produce products having the same thickness.
However, this approach has the following drawback: Although the bank 4 is three dimensional, the camera 6 can only provide two dimensional information. As a result, the reliability of the information obtained through the camera 6 is limited or an accurate bank control is theoretically impossible. Such an incomplete control does not result in producing products having the same thickness.
In addition, the roll gap 3 of a calender of the type as shown in FIG. 8 changes not only with the volume and shape of bank 4, but also with the temperature and plasticity (or viscosity) of the material 8. The change of the roll gap 3 results in the change of the thickness of the product. Therefore, controlling the volume of the bank 4 is not sufficient.
There are other Japanese Patent Applications (Publication Nos. 56-62125, 56-80447 and 59-26220) which also disclose apparatuses for and methods of controlling the roll gap. In these applications, the volume of the bank is detected by a detector which includes an electrostatic capacitor, an optical detector or an image detector, to control the roll gap. However, like the aforementioned prior art, the temperature and the plasticity (viscosity) of the raw material and bank are not considered in controlling the roll gap. Therefore, even if the temperature and/or the plasticity of the raw material changes, the roll gap is not adjusted.